1. Field of the Invention
This invention relates generally to optical mark detecting systems and more particularly to such systems as are used to sense pencil marks or objects on a document.
2. Description of the Prior Art
Optical mark sense detection systems are well-known in which data in the form of pencil marks on documents are recognized by a decrease in transmitted or reflected light when such a mark is encountered. In a typical prior art system, the method used to detect pencil marks on a document might be described as follows. Light from a high intensity source may be directed to the document by means of fiber optics. Light reflected from the document, such as a white or near-white paper material, may be received by other fiber optics and directed to the light-sensitive area of a photo-transistor circuit. A load resistor in that circuit will develop a voltage as a result of the conduction of the photo-transistor, which voltage is a function of the amount of light reflected from the document. If now a pencil mark is encountered by the optics, there will be a decrease in reflected light due to the decreased reflectance of the mark from the plain unmarked document. That is to say, a pencil mark does not reflect light as well as a white or near-white unmarked document. In these prior art devices a voltage threshold was set on a comparator and if, upon comparison, the voltage developed by the photo-transistor decreased below that threshold, the change in the comparator's output would be an indication of a sensed mark. Thus, light directed to a sensor causes the circuit to conduct current as a function of the amount of light received by the sensor. A reference or threshold level is applied to one input of a comparator while the conduction level of the circuit is applied to its other input. When the circuit conduction level is high, the output of the comparator will indicate no mark or object is present. When an object or mark passes the sensor and decreases the circuit conduction to a point equal to or lower than the reference or threshold level, the output of the comparator will change, thus indicating that a mark or object was detected.
One popular prior art system employs a fixed reference or threshold level. If, for example, this reference level is set at 6 volts when conduction in the circuit drops from, say, 10 volts to 6 volts, the system will indicate that a mark or object has been detected. Thus a 40% decrease in conduction has been necessary to cause this system to indicate detection of a mark or object.
Another prior art system has employed a fixed amount of voltage drop from an original voltage level. If, for example, this system has been set to detect a 4 volts drop from an original conduction level of again, say, 10 volts, the reference will again be set to 10 volts minus 4 volts= 6 volts, thus again, a 40% decrease in conduction is required to cause this system to indicate detection of a mark or object.
It is interesting and instructive to examine both of these prior art systems under new conditions commonly encountered in the art so as to point up deficiencies and areas in need of improvement in such prior art systems. In the interest of specificity, assume that the intensity of the light source has decreased from any causes whatsoever such as, for example, decrease in supply voltage, aging, or the like. Assume further that this decrease in light intensity results in a decrease in conduction with no mark or object present from 10 volts down to, say, 8 volts.
Our first prior art system will still have the fixed reference or threshold voltage of 6 volts. It now requires, however, only a 2 volt drop in conduction to cause that system to indicate detection of a mark or object. This is only a 25% decrease in conduction. This increase in sensitivity may be enough to cause the system to indicate the presence of a mark or object when, in fact, none is present.
In the other prior art system considered, in which a specified voltage drop is required, we still require a 4 volt drop for the system to indicate detection of a mark or object. This requirement is now for a 50% drop in conduction from the 8 volts conduction with no mark or object present. This decrease in sensitivity may be intolerable in the system thus described.
Some of these prior art devices have been described in the below listed patents that were brought to the attention of applicant's attorney. It is hoped that this information may be of some help to the Examiner:
1. U.S. Pat. No. 3,600,558 -- "Coded Card and Reading Means" -- Grosbard PA0 2. U.S. Pat. No. 3,639,729 -- "Data Reading Apparatus" --Marshall PA0 3. U.S. Pat. No. 3,639,730 -- "Optical Reader System" -- Higginbotham et al. PA0 4. U.S. Pat. No. 3,820,068 -- "Background Reference Level System and Method for Document Scanners" -- McMillin PA0 5. U.S. Pat. No. 3,870,865 --"Method and Apparatus for Optical Reading of Recorded Data" -- Schneiderhan et al. PA0 6. U.S. Pat. No. 3,894,216 -- "Illumination and Sensor Arrangement for Card Reader" -- Bottles PA0 7. U.S. Pat. No. 3,896,294 -- "Plural Mode Card Reading Apparatus" -- Schisselbauer et al. PA0 8. U.S. Pat. No. 3,904,110 -- "Large Mark Tolerance Card Reader" -- Bottles
It would thus be a great advantage to the art to provide a system in which the reference threshold level is dynamically varied so as to maintain a constant sensitivity under some adverse conditions commonly encountered in such optical mark sense detection systems.
Another great advantage to the art would be the provision of a method of optical detecting by an optical system wherein the reference level is always directly a portion of the level before the object or mark to be sensed was present.
A further desirable advantage to the art would be the provision of a system in which the conduction level in the absence of marks or objects is constantly monitored and stored in a manner by which the stored level increases immediately with any increase in conduction but decreases very slowly when conduction decreases.
Yet another advantage to the art would be a system wherein a preset portion or percentage of the stored level of conduction is used as a reference or threshold to compare to a sensed level of conduction that is not being stored.